The invention relates to a stop motion device for web feed printing presses of the type having at least one web interrupting (i. e. gripping and/or severing) device arranged between the last printing unit and a dryer and able to be triggered by a sensor responsive to a tear in the web.
A stop motion device of this type has been described in the German pre-examination patent specification No. 3,431,686 in which a web clamping device mounted on the dryer is operated by a paper detector switch which is arranged downstream from a dryer or from a cooler following the dryer. Although this position for accommodating the paper detector switch is somewhat where tears are prone to occur in the web, it is not necessarily the case that every tear in the web leads to the web winding itself up on parts of the machinery and thus to the danger of damage thereto. Such a danger is in fact only present if paper web, which has ink on it, runs slack owing to a tear so that the ink causes it to stick to an inking cylinder in the press, that is to say, in the case of an offset litho press, a blanker cylinder, so that the paper winds itself up onto the blanket cylinder. In such a case the web has to be quickly gripped and/or cut in order to prevent damage to the plant. In other cases this is not necessary and only necessitates renewed draw in of the web from the position of severance and resetting of the web gripping and/or cutting device. In the known arrangement with a paper detector switch arranged after the dryer the web gripping and/or cutting device is put into operation every time that is to say without drawing any distinction between the danger of the web winding itself up on a part of the press and or the absence of such a danger. Accordingly the known device is not easy to operate. A further fact is that the sensing of the web takes place downstream from the dryer, a mechanically functioning sliding shoe generally being used which rides on the web and has a very long response time so that if there is a need to take prompt action the web interrupting device is frequently not triggered early enough. The known arrangement is consequently not sufficiently reliable in operation.